- the ascending lunar node (common symbol: ☊) is the one where the moon changes from the southern to the northern side of the ecliptic ("ascends")
- the descending lunar node (☋) is the one where the moon changes from the north to the south side of the ecliptic.
Nodes and eclipses
Eclipses are only possible in the vicinity of a passage of the moon through one of the nodes:
Both types of darkness can occur at both the ascending and the descending node.
- Dragon's head ( Caput draconis , ascending lunar knot)
- Dragon tail ( Cauda draconis , descending lunar knot)
and had a special meaning.
The two points of the lunar orbit with the greatest northern and southern ecliptical latitudes (i.e. the greatest distance from the ecliptic) were called the dragon's belly ( Venter draconis ); they differ from the nodes by 90 ° in length .
The same etymological meaning is in the "ecliptic" (Greek ékleipsis - literally "concealment, extinction"). It can also be found in Sino-Confucian mythology in Xi and He , 羲 和, the heavenly officials for the sun and moon. However, this myth has no known connection with the constellation "Dragon" .
The draconian month
The length of the draconian month is given as 27.21222 days. That's 27 days, 5 hours, 5 minutes and 35.8 seconds.
It is thus 0.1095 days (i.e. 2h 37.6m) shorter than the tropical month - related to the equinox - because the lunar nodes move backwards by approx. 19.3 ° annually in the ecliptic due to the precession of the lunar orbit plane . A complete orbit of the nodes in the ecliptic takes 6798 days or 18.61 years.
The draconian month is used to calculate eclipse cycles , the regular repetitions of solar and lunar eclipses.
Lunar knot and nutation
The lunar nodal drift causes a change in the position of the lunar orbit on an observer's sphere .
- the inclination of the ecliptic (the ecliptic forms an angle of 23 ° 27 ' with the plane of the celestial equator ) and
- the inclination of the lunar orbit in relation to the ecliptic (mean inclination 5 ° 9 ')
to a maximum declination of the moon of 28 ° 36 '(minimum 18 ° 17'). The extremes of this cycle are called the moon turns .
Specifically, the following inclinations of the lunar orbit towards the celestial equator result for the coincidence of a lunar node with the spring or autumn point:
|Spring equinox||Autumn point|
|before: -28 ° 36 '
after: + 28 ° 36'
|before: + 18 ° 17 '
after: -18 ° 17'
|before: -18 ° 17 '
after: + 18 ° 17'
|before: + 28 ° 36 '
after: -28 ° 36'
The gravitational action of the moon, which fluctuates around these extreme positions, causes nutation , a periodic axis fluctuation of the earth with the same duration of 18.613 years, which is therefore called the nutation period.
- Joachim Herrmann : dtv-Atlas Astronomie (= dtv. Dtv-Atlas 3267). 15th, revised and updated edition. Deutscher Taschenbuch-Verlag, Munich 2005, ISBN 3-423-03267-7 .
- Joachim Herrmann: dtv atlas for astronomy (= dtv 30069). 2nd Edition. Deutscher Taschenbuch-Verlag, Munich 1974, ISBN 3-423-03006-2 , p. 51.